Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dye having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, is heated by means of a thermal printing head provided with a plurality of juxtaposed heat-generating elements or resistors, so that dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which has been covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer.
Owing to the fact that the thin support softens when heated during the printing operation and then sticks to the thermal printing head, thereby causing malfunction of the printing apparatus and reduction in image quality, the back of the support (the side opposite to that carrying the dye layer) is typically provided with a heat-resistant layer to facilitate passage of the dye-donor element past the thermal printing head. An adhesive layer may be provided between the support and the heat-resistant layer.
The heat-resistant layer generally comprises a lubricant and a binder. In the conventional heat-resistant layers the binder is either a cured binder as described in e.g. EP 153,880, EP 194,106, EP 314,348, EP 329,117, JP 60/151,096, JP 60/229,787, JP 60/229,792, JP 60/229,795, JP 62/48,589, JP 62/212,192, JP 62/259,889, JP 01/5884, JP 01/56,587, and JP 02/128,899 or a polymeric thermoplast as described in e.g. EP 267,469, JP 58/187,396, JP 63/191,678, JP 63/191,679, JP 01/234,292, and JP 02/70,485.
During printing, a smooth transport of the donor ribbon and the dye-receiving element is required in order to obtain a good density uniformity all over the print. However, when white lines in a flat field of high density are printed parallel to the line of heater elements, the drum transport is hindered and the white line is further elongated up to the edges of the printed image.
This phenomenon occurs in particular when the average printing power of said heat-generating elements exceeds 4.5 W/mm.sup.2. The average printing power is calculated as the total amount of energy applied during one line time divided by the line time and by the surface area of the heat-generating elements. Conventional thermal printers usually operate with a maximum average printing power of 3 to 4.5 W/mm.sup.2. However, to obtain increased print densities and/or faster printing speeds, it is desirable to use an average printing power larger than 4.5 W/nmL.sup.2.
These high printing energies are used in thermal sublimation printers, which for the sublimation (or diffusion) of dye require substantially higher printing energies than thermal wax printers, in which delamination and fusion of the dye later are caused.
It has been suggested to use different types of lubricants to allow continuous transport of the dye-donor ribbon relative to the thermal head.
In EP 153,880 and EP 194,106, phosphoric acid derivatives are used as slipping agents. However, these products give corrosion problems due to the high acidity content. Moreover, these slipping agents hydrolyse the binders of the heat-resistant layer during the coating procedure, storage of the donor ribbon and/or printing procedure.
It has been suggested to neutralise these phosphoric acid derivatives with potassium or sodium hydroxide. However, it is well known that potassium and sodium ions interfere with electronic components such as thermal heads (especially at higher temperatures).
Other well known lubricants are polysiloxanes such as those mentioned in EP 267,469, EP 138,483, U.S. Pat. Nos. 4,738,950, 4,866,028, 4,753 920 and U.S. Pat. No. 4,782,041. Especially useful slipping agents are polysiloxane-polyether block or graft polymers. Although, the above lubricants perform well at constant printing density they give rise to white lines.
Metal salts of long fatty acids are also well-known lubricants (such as mentioned in EP 458,538, EP 458,522, EP 314,348, JN 01/241,491 and JN 01/222,993). White lines are observed, however, when high printing energies are applied.
It has been suggested to use lubricating polymers such as polyethylene waxes or lubricating waxes such as amide or ester waxes in combination with a polysiloxane lubricant in order to prevent this phenomenon.
However, the use of polyethylene waxes in the heat-resistant layer results in a contamination of the thermal head when multiple printing is performed. Low molecular weight waxes such as amide or ester waxes reduce the problem only to a certain extent. At higher printing energies, a white line is still observed.